1. Field of the Invention
This invention generally relates to a piezoelectric gas sensing device including a piezoelectric sensing element such as a quartz crystal microbalance or a surface acoustical wave (SAW) element.
2. Description of the Related Art
In the semiconductor manufacturing industry and in other industrial process and manufacturing fields, a number of systems and techniques have been developed for monitoring a fluid, e.g., a process stream or an ambient environment in a plant facility, for the presence of specific gas components. Applications in which such fluid monitoring is carried out include monitoring of ion implant cabinets for hydride and acid gases, monitoring of process streams to determine the end point utility of a scrubber medium employed for treatment of such streams to remove hazardous gas components therefrom, and monitoring of room air for toxic gas components.
In such fluid monitoring applications, piezoelectric devices have been used for gas sensing. Examples include the use of a surface acoustical wave (SAW) element to sense concentration of a dopant species, as described in U.S. Pat. No. 4,936,877 issued Jun. 26, 1990 to Steven J. Hultquist and Glenn M. Tom for "Dopant Delivery System for Semiconductor Manufacture," and the use of quartz microbalance systems for gas stream monitoring and environmental sensor applications, as described in U.S. patent application Ser. No. 08/678,572 filed Jul. 12, 1996 in the name Glenn M. Tom and Cynthia A. Miller for "Piezoelectric End Point Sensor for Detection of Breakthrough of Fluid, and Fluid Processing Apparatus Comprising Same," U.S. patent application Ser. No. 08/679,258 filed Jul. 12, 1996 in the names of Glenn M. Tom and Cynthia A. Miller for "Piezoelectric Environmental Fluid Monitoring Assembly and Method," and U.S. patent application Ser. No. 08/785,342 filed Jan. 17, 1996 in the names of Glenn M. Tom and Cynthia A. Miller for "Piezoelectric Sensor for Hydride Gases, and Fluid Monitoring Apparatus Comprising Same," the disclosures of which hereby are incorporated herein by reference in their entireties.
In these applications, the piezoelectric element is coated with an affinity coating which is selective for particular gas species. For example, quartz crystal microbalances when used as gas sensing elements are provided with electrode coating materials that are selective for interaction with one or more gas species. Surface acoustical wave elements when employed as gas sensing elements are likewise are coated with materials selective for interaction with a desired gas species, typically as a film of the affinity coating on the top surface of the piezoelectric crystal between the respective electrode structures of the device.
In these applications, the permeability and thickness characteristics of the affinity coatings are critical to the sensitivity and speed of response of the piezoelectric device. In general, the affinity coating (which may comprise a physical adsorbent material or a chemisorbent material) should be highly available to the gas species of interest, to maximize the speed of binding or reaction of the gaseous species with the sorptive or chemically reactive sites in the affinity coating, and to maximize speed of response.
It is known to utilize porous material coatings on a quartz microbalance to provide a high surface area affinity coating, as described in the aforementioned U.S. patent application Ser. No. 08/785,342 filed Jan. 17, 1996 in the names of Glenn M. Tom and Cynthia A. Miller for "Piezoelectric Sensor for Hydride Gases, and Fluid Monitoring Apparatus Comprising Same."
The art has continued to seek improvements in piezoelectric devices for gas sensing applications.
It would be a substantial advance in the art, and is an object of the present invention, to provide an improved piezoelectric device substrate providing an enhanced interaction with the gas species of interest.
It is another object of the present invention to provide an improved surface acoustical wave device of such type.
It is a further object of the invention to provide a quartz microbalance device of such type.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.